1. Field of the Invention
The present invention pertains to tantalum and tantalum nitride films which can be stress tuned to be in tension or in compression or to have a particularly low stress, and to a method of producing such films. These stress tuned films are particularly useful in semiconductor interconnect structures where they can be used to balance the stress within a stack of layers which includes a combination of barrier layers, wetting layers, and conductive layers, for example. The low stress tantalum and tantalum nitride films are particularly suited for the lining of vias and trenches having a high aspect ratio.
2. Brief Description of the Background Art
A typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (.mu.m) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a conductive material onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using a chemical, mechanical, or combined chemical-mechanical polishing techniques. Future technological requirements have placed a focus on the replacement of aluminum (and aluminum alloys) by copper as the conductive material. As a result, there is an increased interest in tantalum nitride barrier layers and in tantalum barrier/wetting layers which are preferred for use in combination with copper.
Tantalum nitride barrier films, Ta.sub.2 N and TaN, have been shown to function up to 700.degree. C. and 750.degree. C., respectively, without the diffusion of copper into an underlying silicon (Si) substrate. Tantalum barrier/wetting films have been shown to function at temperatures of approximately 500.degree. C. It is advantageous in terms of processing simplicity to sputter the barrier and or wetting layers underlaying the copper. Tantalum nitride barrier layers are most commonly prepared using reactive physical sputtering, typically with magnetron cathodes, where the sputtering target is tantalum and nitrogen is introduced into the reaction chamber.
S. M. Rossnagel and J. Hopwood describe a technique which enables control of the degree of directionality in the deposition of diffusion barriers in their paper titled "Thin, high atomic weight refractory film deposition for diffusion barrier, adhesion layer, and seed layer applications" J. Vac. Sci. Technol. B 14(3), May/June 1996. In particular, the paper describes a method of depositing tantalum (Ta) which permits the deposition of the tantalum atoms on steep sidewalls of interconnect vias and trenches. The method uses conventional, non-collimated magnetron sputtering at low pressures, with improved directionality of the depositing atoms. The improved directionality is achieved by increasing the distance between the cathode and the workpiece surface (the throw) and by reducing the argon pressure during sputtering. For a film deposited with commercial cathodes (Applied Materials Endura.RTM. class; circular planar cathode with a diameter of 30 cm) and rotating magnet defined erosion paths, a throw distance of 25 cm is said to be approximately equal to an interposed collimator of aspect ratio near 1.0. In the present disclosure, use of this "long throw" technique with traditional, non-collimated magnetron sputtering at low pressures is referred to as "Gamma sputtering".
Gamma sputtering enables the deposition of thin, conformal coatings on sidewalls of a trench having an aspect ratio of 2.8:1 for 0.5 .mu.m-wide trench features. However, we have determined that Gamma sputtered TaN films exhibit a relatively high film residual compressive stress, in the range of about -1.0.times.10.sup.+10 to about -5.0.times.10.sup.+10 dynes/cm.sup.2. High film residual compressive stress, in the range described above can cause a Ta film or a tantalum nitride (e.g. Ta.sub.2 N or TaN) film to peel off from the underlying substrate (typically silicon oxide dielectric). In the alternative, the film stress can cause feature distortion on the substrate (typically a silicon wafer) surface or even deformation of a thin wafer.
A method of reducing the residual stress in a Ta barrier/wetting film or a Ta.sub.2 N or TaN barrier film would be beneficial in enabling the execution of subsequent process steps without delamination of such films from trench and via sidewalls or other interconnect features. This reduces the number of particles generated, increasing device yield during production. In addition, a film having a near zero stress condition improves the reliability of the device itself.